# CVE-2026-53424: Missing one-time-use enforcement in Samly allows replay of SAML bearer assertions

> Attackers can replay captured SAML assertions to authenticate as other users due to missing one-time-use enforcement in the Samly library.

- **Severity:** Critical 9.1 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 20 Aug 2026
- **Updated:** 21 Aug 2026
- **Also known as:** EEF-CVE-2026-53424, GHSA-5x87-gf34-8ww7

## What is CVE-2026-53424?

CVE-2026-53424 is a critical authentication bypass vulnerability in dropbox samly, published on 20 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Authentication Bypass by Capture-replay vulnerability in dropbox samly allows an attacker to authenticate as the subject of a captured SAML assertion by resubmitting it.

Samly.Helper.decode_idp_auth_resp/3 in lib/samly/helper.ex calls esaml_sp:validate_assertion/2, whose default duplicate detector is a no-op. The /3 arity accepting a DuplicateFun exists in esaml and implements the check, but Samly never calls it and offers no configuration to supply one, so the SAML 2.0 Web Browser SSO Profile requirement that a bearer assertion be used once is unenforced. An attacker holding a valid SAMLResponse obtained from the network, from browser history, or from logs can submit the identical bytes repeatedly until the assertion's NotOnOrAfter passes, each time establishing a session as the assertion's subject.

This issue affects samly: from 0.3.0 onward.

Weakness: CWE-294 (Authentication Bypass by Capture-replay)

## Which versions are affected by CVE-2026-53424?

1 open source package ship an affected version, including samly.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `samly` | Hex | From 0.3.0 | None |

## How do I fix CVE-2026-53424?

No fix has been published for CVE-2026-53424 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-53424 being exploited?

There is no known exploitation of CVE-2026-53424 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-53424 and fix it if it is affected.

What it is: CVE-2026-53424 is a critical authentication bypass vulnerability in dropbox samly, published on 20 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-53424 in the wild and no public exploit code.

Affected packages and fixed versions:
- samly (Hex): affected from 0.3.0; no fix listed

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-53424/ (Markdown: https://insights.sbomify.com/CVE-2026-53424/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-53424
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-53424
- OSV: https://osv.dev/vulnerability/CVE-2026-53424

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-53424/
Summaries are generated and may contain mistakes; the official description above is the record.